1. Field of the Invention
The present invention relates to a method of inspecting a wafer surface. More particularly, the present invention relates to a method for discriminating among types of wafer defects using a wafer surface inspection device which detects multi-channel scattering light from a dark field or defect region of a wafer.
2. Description of the Related Art
Recently, the integration of integrated circuit (IC) memories has improved from 16 megabits to 64 megabits or more. As a result, pattern widths have taken on microscopic dimensions, and microscopic extraneous substances which did not cause problems in the past now act as sources of contamination and possible defects. Accordingly, when manufacturing highly integrated superhigh integration LSI devices such as 64 megabit-dynamic random access memories (DRAMs) or 256 megabit-DRAMs, it is desirable to thoroughly control and thus avoid defects and/or micro-sized extraneous substances on the wafer to thereby significantly enhance yield.
Generally, the size of a defect which can cause problems is a factor of the minimum wiring width of the superhigh integration LSI to be manufactured. For example, it is necessary to control micro-sized defects which have diameters of 0.1-0.2 xcexcm or less in 256 megabit-DRAMs or in devices having capacities more than the 256 megabit-DRAMs, which are implemented according to a design rule of 0.23 xcexcm or less.
Defects which cause problems during the manufacture of superhigh integration LSI devices can be largely divided into two types. One is xe2x80x9ccrystal originated particlexe2x80x9d (COP) defects formed on a wafer surface or inside the wafer during manufacture of the wafer. The other is actual dust or contaminants (hereinafter, called particles) adhering to the wafer surface.
COPs are generated during the manufacture of silicon wafers. Generally, crystal defects called xe2x80x9cas-grownxe2x80x9d defects are introduced in an ingot of single crystal silicon which is pulled according to a so-called Czochralski method (hereinafter, called a CZ method) during the growth of the single crystal silicon. This crystal defect is not removed during crystal cooling and remains in the processed and manufactured wafer. In this state, when wet-cleaning is performed (which is generally used to remove particles adhering to the surface of the wafer), micro pits are formed on the wafer surface since the etching rate is greater at or in the vicinity of the crystal defects on the wafer than at portions of the wafer which are free of such defects. Here, such pits are called COPs.
FIG. 1 is a sectional view showing representations of COPs and particles on a wafer 1. It is known that the COP degrades the electric breakdown characteristic of a gate oxide film on a semiconductor device. Furthermore, the COP acts as a contamination source on the wafer which can cause breaking or shortcircuiting of a circuit pattern. As such, the COP may result in defective products or decreased product quality and reliability.
When a wafer having both COPs and particles is measured by a conventional particle counter or a wafer inspection device, frequently the COPs (which are vacancy-type defects) are mistakenly detected as particles. That is, the COPs are not accurately discriminated from particles which correspond to actual contaminants. However, to properly manage defects and thus improve yield, it is important to accurately discriminate COPs from particles on a wafer. Therefore, it is desirable to quantitatively measure and analyze the states of the COPs and/or the particles on the wafer to permit discrimination thereof.
U.S. Pat. No. 5,640,238, issued to Nakano et al., discloses a conventional technology for discriminating COPs from particles on a wafer. In this patent, a method of inspecting particles comprises the steps of: making a first particle map by particle measurement on a wafer to be inspected; performing a particle removing treatment to remove particles from the wafer; making a second particle map by particle measurement on the wafer which was subjected to the particle removing treatment; and comparing the first particle map with the second particle map. Particles appearing at each immobile point common in both the first and second particle maps are determined to be crystal defects or surface irregularities, and particles appearing in the first particle map but not in the second particle map are determined to be real dust particles or contaminants.
In such conventional technology, however, since a cleaning process must be carried out to remove particles from a wafer, there is a disadvantage in that it takes additional time to monitor the particles on the wafer. Particularly, since the process for removing particles must be carried out even when only COPs which affect yield are present (as opposed to particles which may be dealt with during fabrication), an unnecessary process is redundantly added, thereby increasing processing time. Moreover, when using a wet-cleaning process to remove particles according to the conventional technology, not only do cleaning solutions cause environmental pollution, but the COPs become larger due to an etching effect of the wet-cleaning process. Further, the conventional technology cannot be applied to achieve in-situ monitoring during the manufacture of devices, and the wafers used for the monitoring cannot be reused, thereby decreasing productivity.
It is an object of the present invention to provide a method of inspecting a wafer surface which allows in-situ monitoring during the manufacture of semiconductor devices and which rapidly and accurately discriminates COPs from actual particles on the wafer.
To achieve the object of the present invention, there is provided a wafer surface inspection method for discriminating among types of defects on a wafer according to defect measurements obtained from a wafer inspection system which includes a plurality of dark field detectors. Using the wafer measurement system, it is determined whether first, second and third conditions are satisfied. The first condition is when a size of a defect on the wafer measured by the wafer inspection system is smaller than a limit value denoting a maximum size of crystal originated particles. The second condition is when a correlation between a plurality of defect light intensity values detected by a plurality of dark field detectors of the wafer measurement system satisfies a reference condition. The third condition is when a location of the defect measured by the wafer inspection system is within a vacancy-rich area of the wafer. The type of the defect is then determined to be a crystal originated particle when the first, second and third conditions are all satisfied. On the other hand, the type of defect is determined to be an actual particle when any one or more of the first, second and third conditions is not satisfied. Preferably, the limit value indicating a maximum size of COPs is 0.16 xcexcm, and the wafer inspection system is a scatterometric particle measurement system.
Each of the dark field detectors may include a wide angle photo-multiplier tube for detecting light scattered from the defect at a wide angle and a narrow angle photo-multiplier tube for detecting light scattered from the defect at a narrow angle. In this case, the plurality of defect light intensity values include a defect intensity value SW obtained from the wide angle photo-multiplier tube and a defect intensity value SN obtained from the narrow angle photo-multiplier tube, and the second condition is where an intensity ratio SN/SW of the intensity value SN to the intensity value SW exceeds the reference value.
Alternately, each of the dark field detectors may include a small angle detector for detecting light components scattered forward from the defect at a small angle, a medium angle detector for detecting light components scattered perpendicularly from the defect at a medium angle and a large angle detector for detecting light components scattered backwardly from the defect at a large angle. In this case, the plurality of defect light intensity values include a defect light intensity value SS obtained from the small angle detector, a defect light intensity value SM obtained from the medium angle detector and a defect light intensity value SL obtained from the large angle detector, and the second condition is where both (a) an intensity ratio SM/SL of the intensity value SM to the intensity value SL exceeds a first reference value and (b) an intensity ratio SM/SS of the intensity value SM to the intensity value SS exceeds a second reference value.
According to the present invention, the error rate of the defect discrimination is minimized, inspection of the wafer surface may be achieved by in-situ monitoring during the manufacture of semiconductor devices, and the type of defect on the wafer is rapidly and accurately determined in a nondestructive manner.